BEIJING -- A key measure of air pollutants in China's capital showed a 19 per cent drop in the first three months of the year, local authorities said Wednesday.

Beijing's municipal government said Wednesday that the density of PM2.5 -- harmful particles that are smaller than 2.5 microns in diameter -- was 92.7 micrograms per cubic meter for the quarter, an improvement from 114 in the same period last year. The World Health Organization safety level for an annual average is 10 micrograms, and China's own standards set the safety level at 35 micrograms.

Despite the improvement, Beijing remains one of the most polluted cities in China, according to the environmental group Greenpeace.

Drawing on governmental data, Greenpeace said the density level of PM2.5 in Beijing decreased by 13 per cent to 92.4 micrograms in the first quarter. Zhang Kai, climate and energy campaigner at Greenpeace East Asia said the discrepancy may stem from two slightly different data sets. He said Greenpeace drew on raw, real-time data from 12 governmental monitoring sites while Beijing authorities could have used revised data from more monitoring sites.

"But the general direction and the overall conclusion are the same," said Zhang, noting this is the first time such comparison is possible after Beijing began to release air pollution data.

Beijing and national leaders have vowed to tackle the problem of air pollution.

In its statement, Beijing authorities said the levels of other pollutants, such as PM10, sulfur dioxide and nitrogen dioxide also dropped in the first quarter, by 8.8 per cent, 42.9 per cent and 11.4 per cent respectively. January through March usually is the city's worst period for pollution.

Authorities say the closure of coal-fired factories and suspension of some construction sites have contributed to better air in Beijing.

Greenpeace said it welcomes the transparency but urged Chinese authorities not to relocate pollution to less developed regions.